Two-stage electron multiplier



u P. T. FARNSWORTH 2,151,620-

TWO-STAGE ELECTRON MULTIPLIER Filed Apl "il 26. 1957 nmmm;

INVENTOR,

A $5 I my mum I 78) PH/LO r. FARNSWORTH. Q 321 I RNEYsE Patented June 6, 1939 TWO-STAGE ELECTRON MULTIPLIER.

Philo T. Farnsworth,

Springfield Township,

Montgomery County, Pa., assignor, by mesne I assignments, to Farnsworth Television & Radio Corporation, Dover, DeL, a. corporation of Delaware Application April 26, 1937, Serial No. 138,925

2 Claims.

My invention relates to two-stage electron multipliers, and more particularly to a two-stage electron multiplier which is particularly adaptable for use within a television dissector tube following, broadly, the teaching of my prior patent, No. 1,773,980, entitled Television system.

'Among'the objects of my invention are: To provide a simple D. C. energized electron multiplier; to provide a simple two-stage electron multiplier; to provide an electron multiplier of small size and of the proper dimensions to be used within a television dissector tube; to provide a dissector target containing a two-stage electron multiplier; to provide an improved dissector tube; to provide a means and method for increasing the output of a dissector tube without substantially increasing shot noise; to provide a means and method of multiplying electrons comprising an elemental portion of an electron image in a cathode ray tube; and to provide an electron multiplier with a minimum number of stages but with maximum gain per stage.

My invention possesses numerous other objects and features of advantage, some of which, together with the foregoing, will be set forth in the following description of specific apparatus embodying and utilizing my novel method. It is therefore to be understood that my method is applicable to other apparatus, and that I do not limit myself, in any way, to the apparatus of the present application, as I may adopt various other apparatus embodiments, utilizing the method, within the scope of the appended claims.

Referring to the drawing:

Figure 1 is a longitudinal sectional view of one form of my invention utilizedas a straight electron multiplier, together with an operating circuit diagram.

Figure 2 is a diagrammatic sectional view of a television dissector tube showing the positioning of my invention within the envelope.

Figure 3 is a sectional view of the multiplying structure utilized in the tube of Figure 2, enlarged to show greater detail.

In my patent above referred to, an electron image is formed within an envelope by projecting a light image on a photoelectric cathode and maintaining the electronic emission therefrom in parallel array so that an electron image is formed, each elemental area of which corresponds in electron density to the light on the elementary area from which the electrons were emitted. An aperture is placed in the path of this electron image, this aperture being of elemental dimensions, thus allowing only electrons from a single elementary area to pass through the aperture at any one time, and thereafter be collected. The electron image is then oscillated in two dimensions so that the entire electron image is scanned across the aperture. Thus, during the scanning cycle, all elementary areas of the electron image pass through the aperture and are collected to form a train of television signals which may then be amplified and transmitted in a manner well known in the art.

I have found that it is greatly desirable to increase the output of the dissector tube, and the present application deals with the use of a twostage electron multiplier operating on a secondary emission principle, and with a multiplier structure which can be made small enough to be placed Within a shield or cylindrical finger, itself sufiiciently small so that it may be placed in the path of the light image being projected on the photoelectric cathode without interfering with the operation of the device. The electron multiplier utilized herein, in conjunction with the dissector finger, operates broadly on the same principles outlined in my Patent 'No. 2,037,711 of April 21, 1936, for Method and apparatus for television, but herein I utilize only a two-stage generation of secondaries, and utilize a simple three-electrode structure having two opposed surfaces capable of generating secondary electrons at a ratio greater than unity, and an anode therebetween, one of the surfaces only being exposed to the electrons-in a single elementary area of the electron image.

A more detailed understanding of my invention maybe had by a direct reference to the preferred embodiments shown in the figures.

In Figure 1 I have shown the electron multiplier of my present invention utilized as a straight electron multiplier of signal inputs. Here, an envelope l is provided with a side stem 2 carrying a thermionically emissive gun cathode 3, a' gun grid 4 and a gun anode 5, these three electrodes representing any form of the well known electron gun. The electron stream issuing from this gun is projected against an angular multiplier cathode 6, the latter having the surface against which the electrons are projected sensitized to produce secondary electrons at a ratio greater than unity. In this regard I have found that secondary electrons at a ratio of 10 or 12-1 may be obtained by forming the multiplier cathode 6 of silver, oxidizing the silver, andexposing the oxidized surface to metallic caesium until maximum secondary emissionis obtained.

However, any surface having the properties desired may be used.

The angular cathode 6 is supported on a lead I sealed through an end stem 9, and is surrounded by a cylindrical shield I0. At one end of the cylindrical shield I is positioned a multiplier anode I I, and back of the multiplier anode II is positioned an opposed multiplier cathode I2. The angle of the angular multiplier cathode 6 is such that secondafiis emitted therefrom are directed toward the second cathode I2 after passing through anode II, and the cylindrical shield I0 creates a focusing field, when energized, to prevent divergence of the electrons during their travel toward the second cathode.

The circuit diagram includes a gun cathode source I4 connected to the gun cathode 3, and a gun anode source I5 providing a potential sufficient to project source electrons against multiplier cathode 6. The input signal is impressed upon gun grid 4 through input line I6.

Various potentials are placed upon the multiplier electrodes, preferably through a voltage divider II, the potential thereon being obtained by passing current therethrough from multiplier source I8. The various multiplier electrodes are connected so that angular multiplier cathode 6 has the lowest potential, cylindrical shield III has the next higher potential, opposed cathode I2 has the next higher potential, and multiplier anode II has the highest potential, the current picked up by the anode II being passed through an output resistor from which the output may be taken through output lead 2 I.

In operation, initial electrons are emitted from gun cathode 3 controlled by gun grid 4, and projected against angular multiplier cathode 6. Upon impact with this cathode, secondary electrons are generated at the ratio, say, of 10 to l, and the augmented electron stream is then accelerated, under the influence of multiplier anode I I and the opposing cathode I2, to pass through the :apertures in the anode screen II and impact the opposing cathode I2 where additional secondary electrons are generated, inasmuch as I prefer to form this electrode also of silver and sensitize it as described for the first cathode 6. The stream is thus again augmented by secondary emission at the ratio of 10 to 1, and the emitted secondaries are drawn to and collected by anode screen I I.

In order that this action may take place, I prefer to have the potential of anode II only a sufiicient amount above that of cathode I2 so that the emitted secondaries will be collected rather than accelerated back toward the first cathode 6. In this respect, in one embodiment which has been built, voltages were used as follows:

Volts Gun anode 5 300 First multiplier cathode B 500 Cylindrical shield I0 600 Second cathode I2 1,000 Multiplier anode II 1,090

Thus, it will be seen that for every electron impacting the angular multiplier cathode 6, approximately one hundred electrons are collected by multiplier anode II, and the collected electrons will be a function of the electrons entering the cylindrical shield II] to impact the multiplier cathode 6, as controlled by the grid I.

In Figures 2 and 3 I have shown this form of multiplier mounted in a television dissector tube. In this construction the image of an object 24 is focused on a planar photoelectric cathode 25 through a lens system 26. An electron image is initiated by the photoelectric action of the cathode, and this image is accelerated toward the opposite end of the envelope, the electrons in the image being maintained in parallel array by the use of a magnetic field parallel to the axis of the tube and produced by the action of a focusing solenoid 21 energized by focusing source 29. Electrostatic focusing means is, of course, a full equivalent.

At the opposite end of the tube from the oathode 25 is positioned an anode finger, shown more in detail in Figure 3. This anode finger comprises an external anode shield comparable to shield II) in Figure 1, which has a scanning aperture 3I therein facing cathode 25. The cylindrical shield 30 is fixed to the envelope by being fitted into an envelope arm 32, and an external lead 33 is brought through the envelope wall. Inside the shield 30 is positioned the angular cathode 6, the multiplier anode I I and the opposing multiplier cathode I2, in exactly the same order as has been described for the device of Figure l, with the angular cathode 6 positioned to receive electrons passing through the scanning aperture 3|.

Each of the multiplier electrodes is provided with a lead passing through the envelope wall, and the electrodes are all supplied from a potential source I8. In this instance, the cathode 25 is at the lowest potential and is usually grounded. The angular multiplier cathode 8 is at the next highest potential, the tubular shield still higher, the opposed multiplier cathode I2 the next highest, and finally, the multiplier anode II being at the highest potential and connected to the source I8 through an output resistor 20 so that the output may be taken from lead 2|, as described for the device of Figure 1.

The electron image is oscillated in two direc-- tions across the scanning aperture by means of scanning oscillators 35 and 36 energizing scanning coils 3'! and 38, preferably with a sawtooth waveform, or by equivalent means. Thus, each elementary area of the electron image is passed across the aperture 3I, and electrons from each elementary area in succession pass through the aperture and impact the angular cathode 6, as shown by the electron. path line 40 in Figure 3. secondaries are generated by the impact of these primaries, are accelerated through anode II to impact opposing cathode I2, and there generate additional secondaries which are collected by anode II. A greatly increased output of the dissector tube is thus obtained over that which would be obtained simply by collection of elec trons passing through the aperture 3 I There is a very distinct reason for utilizing anode II as the final collector instead of having the electrons collected by cathode I2. For example, there are electron multiplier structures where the electrons are passed through successive screens, generating secondaries at each passage through the screens, because of the fact that certain of the electrons will impact the screen material, whereas others will pass directly through. If this were done, however, in a. device of the sort described, particularly in a television dissector tube, it is obvious that certain electron components would go through the screen. II to be collooted by the electrode I2 without being multiplied by impact with screen II, whereas others would impact screen I I, generate secondaries, and these secondaries would then be pulled through the screen to be collected by electrode I2.

However, it is obvious that the two groups of electrons representing a single initial stream would not be likely to arrive at collector l2 at the same time, and therefore a shot effect would be entered into the operation of the device. With the herein described mode of operation, the electrons generate secondaries only on solid surfaces; all those electrons starting at the same time are collected at the same. time and after an equal multiplication. Therefore, the advantage of utilizing the opposed surfaces with the intermediate collector becomes apparent. Furthermore, with the utilization of only two stages, as shown, the multiplication per stage can be made extremely large without the intervention of space charge effects, and with such large multiplications an overall multiplication of 100 to 1 can be readily obtained, and this multiplication is sufficient to greatly reduce the necessity for extreme sensitivity in the following amplifiers. Thus, an efiicient output is obtained, with a minimum of shot effect and a minimum of noise, together with faithful transformation of light values into signal values.

I claim: I

1. An electron discharge device having an envelope containing a plurality of surfaces capable of emitting secondary electrons at a ratio greater than unity upon electron impact therewith, and

a photoelectric surface capable of emitting when illuminated, an electron stream of electron image cross section, means for selecting a portion of said stream of elemental dimensions, means for directing said portion against one of said surfaces to produce secondary electrons therefrom, means for directing the produced secondary electrons to another one of said surfaces, said first surface being positioned ata angle to said stream portion and said second surface at right angles to the path of secondary electrons emitted from said first surface.

.2. An electron multiplier comprising an envelope containing means for producing a beam of electrons, a hollow elongated conductor in the path of said beam and having an aperture for admitting electrons from said source, an electrode capable of emitting secondary electrons upon electron impact therewith positioned within said conductor in the path of electrons passing through said aperture, said electrode being positioned at substantially a 45 angle to the path of electrons entering said conductor and to the axis of said conductor, a secondary electron emissive electrode in the path of secondary electrons emitted from said first electrode, and an apertured electrode between said secondary emitting electrodes.

PHILO T. FARNSWORTH. 

